Actually etiquette books will tell you that your child should be II, but they will also tell you that RSS Jr should now NOT be "Jr" because the original "Sr" is deceased. The original idea of "correctness" is that when someone died everyone else moved up a notch; Jr becomes Sr, III becomes Jr, IV becomes III.

Now almost no one really does it that way because in reality it would be too confusing in terms of signing legal documents and other issues. But that's the way they used to tell you was "correct."